The present invention relates to a security paper with authenticity features in the form of luminescing substances on the basis of host lattices doped with rare earth metals.
Under the term "security paper" are to be understood here bank notes, check forms, shares and stamps as well as passes, credit cards, check cards, passports, air tickets and other certificates and documents.
Rendering security paper secure against forgery by means of luminescing substances has already been known for a long time. Already in German Patent Specification 449,133 from the year 1925 and German Patent Specification 497,037 from the year 1926 the introduction of luminescing substances into security papers is described, wherein the luminophores are excitable with ultraviolet or other invisible rays and emit in the visible region.
In U.S. Pat. Nos. 3,473,027 and 3,525,698 luminophores and their use as coding dyestuffs on the basis of host lattices doped with rare earth metals, which if appropriate can be co-activated, are described, in which the excitation can take place in the UV-region and shortwave visible region and the emission in the visible or IR-region, wherein the emission in the IR-region finds use in broadening the usable spectral region.
The co-activated ytterbium-erbium rare earth metal doped luminophores described in German Offenlegungsschrift 2547768 are excited in the IR-region and emit in the visible region.
The use of luminophores for rendering data carriers secure is further described in German Offenlegungsschrift 1599011 as well as in German Offenlegungsschrift 2903073, wherein the luminescent materials there described are excited and emit in the IR-region.
In the patent literature and the scientific literature a very large number of various rare earth metal luminophores are described which as single crystals are suitable for solid body lasers and other purposes. For example attention can be directed to U.S. Pat. Nos. 3,447,851 and 3,480,877 in which crystals with garnet structure are described for laser techniques and for other purposes, but the rendering of security papers secure with luminophores is not mentioned.
From the state of the art concerning rendering security papers secure with luminescing substances it can be concluded that the excitation of the luminophores takes place in the non-visible region i.e. in the UV- or IR-region while the emission in the visible spectrum is either desired or not seen as damaging.
The luminophores are in security papers included as paper additions, as paper insertions, for example as mottling fibres or security threads, or in printing inks.
It has emerged that in rendering security papers secure with rare earth metal luminophores difficulties arise on account of their properties described in what follows. In more recent publications generally "data cards" are described i.e. generally multi-layer security papers in which these difficulties can be avoided e.g. by thick screen printing layers, foil embedding or the like.
Difficulties in rendering security papers secure, particularly bank notes, using rare earth metal luminophores arise from their particle size. In the publications already noted above namely U.S. Pat. No. 3,473,027 and German Offenlegungsschrift 2547768 particle sizes of a few .mu.m upwards are noted. For normal printing pigments, however, particle sizes below 1 .mu.m are necessary. Normal previously used rare earth metal luminophores on comminution to below a certain particle size no longer display sufficient effectiveness. They must accordingly be introduced in large quantities; this gives rise to high costs and often leads to insoluble technological problems, since for this the limit of loading of the printing ink with additional materials has to be exceeded.
For avoiding these difficulties in relation to particle size, partly soluble organic rare earth metal luminophores are described, which, however, because of their nature do not have the solvent fastness necessary for bank note printing.
In the rendering of security papers secure up till now particular value has been laid on the fact that on excitation in the UV- or IR-region luminescence arose in the visible region or in the near IR-region easily accessible with commercial image converters. In automatic authenticity checking of security papers however an additional security factor is present if the security feature is not visible or it is not possible to make it visible with conventional means.
In German Offenlegungsschrift 1599011 for masking inscriptions covering with a foil has already been proposed. Apart from the fact that the foil itself is visible and thereby directs attention particularly to the place of the inscription, covering with foils in the case of bank notes and similar security papers is inappropriate.